1. Field of the Invention
The present invention relates to a valve system of an overhead-camshaft-type internal combustion engine, and an engine incorporating same. More particularly, the present invention relates to a reinforced cam holder which pivotally supports a camshaft of the valve system.
2. Description of the Background Art
There is a known valve system of an overhead cam-shaft-type internal combustion engine in which a cam holder, which pivotally supports a camshaft by both oppositely-facing support walls having lower sides thereof joined to each other by a bottom wall, is fastened to a cylinder head above a combustion chamber. An example of such valve system of an overhead cam-shaft-type internal combustion engine is disclosed in the Japanese Patent Document JP-A-2002-122007.
The overhead-camshaft-type internal combustion engine disclosed the Japanese Patent Document JP-A-2002-122007 is a single-cylinder 4-cycle internal combustion engine. The internal combustion engine is mounted on a motorcycle such that a crankshaft of the internal combustion engine is directed in the vehicle width direction (lateral direction). A cam holder, fastened to a cylinder head above the combustion chamber, has a structure in which left and right oppositely-facing support walls having lower sides thereof joined to each other by a bottom wall pivotally supports the camshaft which is directed in the lateral direction, and fastening boss portions formed on front and rear side portions of the left and right support walls are fastened to the cylinder head using bolts.
The left and right support walls of the cam holder pivotally support not only the camshaft but also a rocker arm shaft by rocker bearing portions arranged in front of the left and right support walls. The rocker arms which are pivotally supported on the rocker arm shaft transmit the rotation of exhaust cam robes of the camshaft for open/close driving of exhaust valves, and an intake valve is directly opened or closed by an intake cam robe of the camshaft.
The cam holder which is fastened to the cylinder head is arranged above a portion of the cylinder head which constitutes a combustion chamber. Since the portion which constitutes the combustion chamber repeats the expansion and the contraction due to a change of pressure in the combustion chamber, the cam holder whose front and rear side portions of the left and right support walls are fastened to the cylinder head using bolts repeatedly receives a force which widens or narrows a distance between the left and right support walls. Hence, the cam holder is likely to be deformed due to such repetition of widening or narrowing of the distance between the support walls.
Accordingly, to overcome such drawback attributed to the repetitious deformation, a wall thickness of the portion of the cam holder to which a stress is applied is increased to ensure the rigidity of the cam holder. In this case, however, it is difficult to realize the reduction of weight of the cam holder.
Here, in the cam holder, a side wall is formed between rear side portions of the left and right support walls. Hence, the rigidity of the cam holder between the rear side portions of the left and right support walls is high so that the deformation of the cam holder is hardly generated between the rear side portions. However, the deformation of the cam holder attributed to the repetitious widening or narrowing of the distance between the support walls is easily generated between front side portions of the cam holder.
Further, the rocker arms are supported between the front side portions of the left and right support walls and hence, a force which acts on the rocker arm is applied to further widen or narrow the distance between the front side portions of the left and right support walls. Accordingly, the deformation attributed to widening or narrowing of the distance between the support walls is further easily generated in left and right front side portions.
In order to prevent (or minimize) the generation of the deformation attributed to widening or narrowing the distance between the support walls, it is necessary to ensure the rigidity by increasing a thickness of the portion of the cam holder which is influenced by a stress and hence, it is more difficult to realize the reduction of weight of the cam holder.
The present invention has been made to overcome such drawbacks. Accordingly, it is one of the objects of the present invention to provide a valve system for an overhead-camshaft-type internal combustion engine which can prevent the deformation of a cam holder for enhancing the durability of the cam holder and realizing the reduction of weight of the cam holder.